Due to the modern diet structure and life style, the population of diabetic patients increases year by year in many countries worldwide. Nowadays, there are 50 million patients in China, 60 million in India, 18 million in the USA and 6 million in Japan, respectively.
There are two main types of diabetes: insulin dependent diabetes (type I diabetes) and non-insulin dependent diabetes (type II diabetes). The cases with type II diabetes account for more than 90 percent of those with the disease. Patients with type II diabetes have various symptoms, such as postprandial insufficient insulin secretion, time delay of insulin secretion, high blood sugar levels and many others. The peripheral insulin acceptor in obese type II diabetic patients shows a decrease in the insulin sensitivity, thus resulting in elevated blood sugar levels with a high insulin level in the blood and a high hemoglobin (HbA1c) level of more than 8% (the non-diabetic range is 4-6%). As a result, the diabetic complications, such as heart disease and kidney failure, etc. occur. So the key to effectively treating type-II diabetes is to decrease the blood sugar level.
Nowadays, drugs used to treat diabetes fall into six broad categories: including the insulin secretagogues, such as sulfonylureas and Meglitinides; and the insulin non-secretagogues, such as insulin, alpha-glucosidase inhibitors, biguanides and Thiazolinediones. However, as shown in one tracking research report on thousands of type II diabetic patients by the UK Prospective Diabetes Study (UKPDS) for six years, none of the six classes of drugs mentioned above are effective on type II diabetic patients. They have all failed to prevent pancreas beta cells from incessant deterioration and have failed to decrease the HbAlc level, prevent diabetic complications, such as heart disease and kidney failure. Therefore, it is necessary to develop novel drugs for type II diabetes therapy.
In 1995, U.S. Pat. No. (5,424,286) for Exendin 4 was issued. Exendin 4 was isolated from the saliva of the Gila monster (Helode Suspectum) that lives in southwestern United States. This 39-amino-acid polypeptide shares 40% identity to glucagon-like peptide 1 (GLP-1) at the amino acid sequence level.
It was reported that Exendin 4, an analogue of GLP-1, binds to the GLP-1 receptor. Exendin 4 stimulates the proinsulin synthesis and insulin secretion, therefore decreasing the blood sugar level. Exendin 4 continues to act until blood sugar returns to normal level. Exendin 4 is safe and effective because it avoids stupor and shock due to hypoglycemia. The Exendin 4 decreases the HbA1c level, increases the beta cell amount, enhances the sensitivity of insulin receptors in the patients with type II diabetes, and inhibits the secretion of glucagons, etc. The commercially available Exendin 4 called Byetta was approved for the market by FDA in April 2005. (Refer to: Diabetes (1997) 46 433-439; ibid (1995) 44 1249-1258; Ibid (2002) 51 2796-2803; Ibid (1994) 53 2397-2403; Diabetes Care (2002) 25 330-336; Ibid (2000) 23 64-69; ibid (2004) 2 2623-2635; JAMA (2002) 287 373-379; N. Engl. J. Med. (2002) 346 393-436; Lanced (1998) 352 837-853; Diabetes Endocrinology (2005) 146 (4) 2069-2070; J. Clin. Endocrinology Metab (2004) 89 3469-3473.)
Efforts have been made to modify the Exendin 4 polypeptide by many researchers, aiming at obtaining more variants that are more effective and convenient to prepare and that provide more alternatives of Exendin 4. A truncated Exendin 4 polypeptide was announced in CN1227567A, which is comprised of 30 amino acid residues with Arg or Tyr at C-terminus. The Lilly Company in USA developed a series of GLP-1 analogues, capable of treating diabetes safely for the long term (Refer to WO02047716A). However, these examples mentioned above are quite limited in that they are only evaluated in vitro by the capacity of binding GLP-1 receptor, the insulin secretion amount of islet cell tumor and the producing amount of cyclic AMP (cAMP) (Refer to: J. Biol. Chem (1997) 272 21201-21206; Regulatory Peptides (2003) 114 153-158; Trend in Pharmacological Sci. (2003) 24 377-383; WO 03011892A).
In order to overcome the disadvantages of existing technologies, the inventors have been striving for a novel C-terminal truncated Exendin 4 polypeptide fragment with Pro at its C-terminus. It is notable that polypeptide with this structure shortens the peptide chain by about ¼ and facilitates production, but provides a new alternative for diabetes treatment. Furthermore, the truncated Exendin 4 can effectively resist carboxypeptidase, and maintain its hypoglycemic activity for a longer period.